The present invention relates to a mobile communication controlling apparatus and an inter-frequency handover control method that allow a mobile station to correctly perform inter-frequency handover even in the real-time media servicing environment.
Third generation mobile systems, which conform to the proposed IMT-2000 standards, are discussed in the 3GPP (the Third Generation Partnership Project) Technical Specifications and Technical Reports.
One of the proposed IMT-2000 standards for third generation mobile networks is a WCDMA based standard. In WCDMA, the same frequencies are reused repeatedly in the cells of a cellular system, and accordingly, inter-frequency handover is unnecessary in the same cellular system. However, in considering concomitance with existing systems, handover between different carrier frequencies (including handover to another cell using a different frequency in the same system and handover to a cell of another system) is required. In view of such circumstances, the 3GPP proposes compressed mode transmission, which is a technique for realizing inter-frequency handover. See 3rd Generation Partnership Project Technical Specification Group Radio Access Network, 25.211 Physical channel and mapping of transport channels onto physical channels, September, 2002.
Explanation is made of the compressed mode. When a mobile station performs inter-frequency handover, a certain time period for switching frequencies and establishing synchronization is required. For this reason, in a portion of the time frame of the active radio link, transmission is suspended (i.e., signal transmission from a base station is temporarily off), and during which period, a different carrier frequency transmitted from a candidate radio base station is monitored. The data of the suspended time period are transmitted at a high rate as compressed frames during the next active period of time. The transmission rate and the link quality are maintained by increasing power. The compressed mode is a function of inter-frequency measurement on frequency bands of other cells for carrying out inter-frequency handover.
FIG. 1 illustrates an example of compressed mode transmission. The timing of shifting to the compressed mode is determined by the network. The network transmits a parameter set required in the compressed mode to the mobile station. In the compressed mode, no data transmission to the mobile station is carried out during the transmission gap. In the compressed mode frame, transmission power is temporarily increased in order to prevent degradation of communication quality (such as the bit error rate (BET) or the block error rate (BLER)) due to fall of the gain during the suspended time period. The transmission gap can be repeated in the compressed mode. In addition, the type of the compression mode (defined by the number of time slots in a transmission gap, the time interval between transmission gaps, the number of repetitions of transmission gap, and other factors, all of which are given as variables) can be changed in response to a measuring request.
FIG. 2 shows an example of the uplink compressed mode frame format. In the uplink frame, the data channel (illustrated in the upper line) and the control channel (illustrated in the lower line) containing the pilot bits are I/Q multiplexed for each radio frame, and a transmission gap is inserted between the transmission frames.
FIG. 3 shows an example of the downlink compressed mode frame format. In the downlink frame, the data channel and the control channel are time-multiplexed. There are two types of downlink frame formats in the compressed mode. In type A compressed mode frame format, priority is given to maximizing the length of the transmission gap. In type B compressed mode frame format, priority is given to power control, and a transmission power control (TPC) slot is inserted in the transmission gap.
In the compressed mode, compressed frames are set to occur periodically because the data to be transmitted in a 10-milisecond radio frame have to be packed in a radio frame including the transmission gap. There are three schemes applicable to the compressed mode, as shown in Table 1.
TABLE 1SCHEMEOUTLINECOMPRESSED MODE BYThe number of transmission bits isPUNCTURINGreduced by a function of ratematching (puncturing). Thespreading factor in the ordinarymode is used in the compressedmode.COMPRESSED MODE BYThe spreading factor (SF) isREDUCING SPREADING BY 2changed such that the same number(SF/2)of bits as in the ordinary modecan be transmitted in the timeslot other than the transmissiongap.COMPRESSED MODE BYThe transport format combinationsHIGHER LAYER SCHEDULING(TFCs) are limited by higherlayers in accordance with thenumber of bits transmittable inthe time slot other than thetransmission gap. The SF in theordinary mode is used in thecompressed mode.
Meanwhile, JP 2003-78936A discloses a technique for reducing degradation of communications quality by reducing the compressed mode transmission as much as possible. In this publication, the radio network controller (RNC) instructs the mobile station to carry out the compressed mode only when a predetermined condition is satisfied. Thus, the compressed mode is implemented intermittently, unlike the continuing or periodic execution of the compressed mode.
With the above-described conventional techniques of CDMA mobile communications systems using multiple carrier frequencies, when a mobile station is going to carry out handover, the transmission mode is shifted to the compressed mode for allowing the inter-frequency measurement at the mobile station, while signal transmission from a base station is temporarily suspended. Accordingly, the amount of transmittable data is reduced in the meanwhile, and data transmission delays occur. For this reason, the compressed mode is unsuitable for streaming services (for downloading data and immediately reproducing the arriving data), such as Internet broadcasts, that do not accept variation or fluctuation in delay.